Position-measuring devices for measuring the angular position of a shaft are described in a multitude of publications. Such position-measuring devices are referred to as rotary encoders. Moreover, position-measuring devices designed with which, in addition to the angular position of the shaft, it is also possible to measure the number of revolutions executed by the shaft are referred to as multiturn rotary encoders.
In principle, two design approaches are used for realizing the multiturn unit, e.g., the unit for determining the number of revolutions executed by the shaft, first of all, multiturn units based on gears, and secondly, multiturn units based on counters.
Counter-based multiturn units ascertain the number of revolutions executed by a shaft by counting the revolutions of a code carrier which is driven directly by the shaft, and therefore executes the same number of revolutions as the shaft to be measured. Located on the code carrier is a code which is scanned by a scanning unit. Based on the position signals ascertained by the scanning unit, counting signals are generated in counter electronics for a counter which counts, as a function of the direction of rotation, the number of complete revolutions of the code carrier, and therefore of the shaft.
In order to store the counter reading of the counter even when the main power supply is switched off, for example, when the machine in which the multiturn rotary encoder is operated is switched off, and in addition, to maintain the counting function, counter-based multiturn units are frequently equipped with a battery which takes over the energy supply for at least the multiturn unit of the rotary encoder upon loss of the main power supply.
For example, European Patent No. 1 462 771 describes a multiturn rotary encoder having a counter-based multiturn unit which is operated with a battery when in the switched-off state.
German Published Patent Application No. 10 2008 015 837 describes a position-measuring device based on an inductive scanning principle that has a battery operation. This position-measuring device is also suited to be implemented as a multiturn rotary encoder.
For safety-related reasons, it may be necessary to measure the number of revolutions of a shaft redundantly, for example, when a faulty measurement may lead to damage of a machine on which the multiturn rotary encoder is operated, or may even result in danger for the operating personnel of the machine.
Therefore, German Published Patent Application No. 10 2009 029 431 describes a multiturn rotary encoder which is equipped with two mutually independent multiturn units. A comparison of the measured values of the multiturn units makes it possible to check the performance reliability of the multiturn rotary encoder at all times. It is therefore suitable for use in safety-related applications.
However, if two counter-based multiturn units are used in such a multiturn rotary encoder, each of which is supplied with energy from a battery for operation in the switched-off state, then in rare cases, fault conditions may still arise which have the same effect on both multiturn units, and therefore cannot immediately be discovered.